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How does a transition metal oxide become more acidic as the metal oxidation number increases?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transition metal Modern version of the periodic table of the elements (printable).

The acidity of transition metal oxides increases as the metal's oxidation number rises because of the changing nature of the chemical bonding within the compound.12

In lower oxidation states (+1, +2, and +3), transition metal oxides are typically ionic compounds consisting of discrete metal cations and oxide anions. These ionic oxides generally exhibit basic behavior, reacting with aqueous acids to form salts and water.1

As the oxidation number increases to +4, +5, +6, and +7, the bonding character shifts from ionic to covalent. These higher-oxidation-state oxides contain covalent metal-oxygen bonds. The increase in oxidation number enhances the electron-withdrawing power of the metal center, which influences the polarity and reactivity of the metal-oxygen bond. Consequently, these compounds transition from being basic to amphoteric (at +4) and finally to acidic (at +5, +6, and +7), where they react with bases to form salts. For instance, among manganese oxides, MnO (+2) is the least acidic, while Mn₂O₇ (+7) is the most acidic.1